Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET) is widely used as a packaging and label material for a variety of products, including foodstuffs.
Because the surface of oriented PET film is hydrophobic, it is not readily receptive to coating or printing. In many packaging and label applications it is advantageous that the film be receptive to ordinary flexographic and rotogravure printing inks.
In an attempt to remedy this deficiency, special inks have been devised that adhere directly to the surface of PET film. However, many such inks are based on formulations of organic solvents, resinous binders and other ingredients which are costly, difficult to handle and hazardous for use in the manufacturing plant.
Another approach for improving the printability of PET film is to apply a primer coating which adheres to the film and is receptive as well to printing inks applied thereto. Examples of such primer coatings include polymeric compositions based on vinylidene chloride terpolymers such as disclosed in U.S. Pat. No. 2,698,240 or water dispersible copolyester primers such as disclosed in U.S. Pat. No. 4,252,885. The problem with this approach to enhancing ink adhesion to PET film is that the film is in many instances used as a food packaging material and consequently the relative toxicity of the primer layer becomes important. Though polyester film per se is approved as a food packaging material by most Governmental agencies, including the U.S. Food and Drug Administration, polyester film primed with certain polymeric primer coatings such as acrylics, vinylidene chloride-containing polymers and many other similar primers raise toxicity problems because of potential migration of said primer material into foodstuff with which it is in contact. In addition, the poor reclaimability of these coated polyester films is a further problem.
It is known to coat polyester film with aqueous dispersions of polyesters or copolyesters containing free functional acid groups in order to produce light-sensitive photographic films, such as is disclosed in U.S. Pat. Nos. 4,252,885 and 4,304,851. In British Pat. No. 1,465,496 it is disclosed to coat polyester films with antistatic matte compositions comprising a polymer or copolymer in order to enchance receptivity to ink. It is also known to improve the printing performance of polyester films by treating the film with a stable polyester base layer and an overlayer of an acrylic or methacrylic polymer or copolymer in a volatile liquid, as is disclosed in British Pat. No. 1,572,345. British Pat. No. 1,371,855 discloses making a polyester film more receptive to printing inks by coating with a primer layer comprising an interpolymerized condensation resin derived from a monoaldehyde, 5 to 25 % by weight of the interpolymer of acrylamide or methacrylamide and at least one other unsaturated monomer, the condensation taking place in the presence of an alkanol containing from one to six carbon atoms. European Publication No. 0029620 teaches a PET priming layer which is based on an aqueous dispersion of the condensation product of ethylene glycol and a mixture of terephthalic acid, isophthalic acid and a salt of sulfoisophthalic acid, which primer is useful for improving the adherence of gelatin-based photosensitive coatings.
None of the prior art disclosures, however, are successfully addressed to the problem of the enhancement of the printability of polyester film and the solution of that problem by the technique hereinafter disclosed.
Accordingly, it is an object of this invention to provide an oriented, self-supporting polyester film material having enhanced receptivity to flexographic or rotogravure printing inks.
Another object is to provide a primer coating for oriented polyester film material which can safely and easily be applied in the plant during production of the film.
A further object is to provide a primer material for biaxially oriented polyethylene terephthalate film material which renders such film generally receptive to flexographic or rotogravure printing inks for packaging applications.